Reduction of sludge deposition from crude oils



March 27, 1962 P. MEYER ET AL REDUCTION OF SLUDGE DEPOSITION FROM CRUDE OILS Filed June 14, 195E Unite States Patent 3,027,319 REDUCTION OF SLUDGE DEPOSITION FROM CRUDE OILS Percy Meyer and Graham Neville Grifliths, London, England, assignors to The British Petroleum Company Limihted, London, England, a British joint-stock corpora- Filed June 14, 1955, Ser. No. 515,442 Claims priority, appiication Great Britain June 29, 1954 7 Claims. (Cl. 208--370) This invention relates to a process for reducing the sludge-forming tendencies of crude petroleum oils.

It is well known that on storage certain crude oils of paraifinic or paralfinic-asphaltic type deposit a wax-containing sludge, which represents a loss of material and which means that tanks on land or on board ships have to be cleaned out from time to time.

In the specification of co-pending application Ser. No. 476,264, now US. Patent No. 2,951,037, granted August 30, 1960, there is described a process for treating crude oils so as to reduce their sludge-forming tendencies. The process consists essentially in heating the crude oil from a low temperature to an elevated temperature which varies with the source of the oil. Oils which are so treated deposit little or no sludge on subsequent cooling and storage. By the expression low temperature, when used above and throughout the rest of this specification, we mean a temperature below 100 F. and preferably not higher than 70 F. The elevated temperature, referred to above and throughout this specification, can be determined for the crude oil from any particular field by the following test. A sample of the crude oil is heated in a pressure tight bomb to 212 F. and is maintained at this temperature for one hour. It is then cooled to 60 F. (If the sample has been freshly taken from a well separator or stabilizer the heating step can be omitted since its purpose is merely to eliminate any previous thermal history of the sample.) The sample (at 60 F.) is then divided into a number of portions which are heated for one hour to various temperatures within the range 60 F. to 212 F., preferably at narrow intervals say of 15 F. The portions are then cooled to 60 F. and after standing for 12 hours are centrifuged for 15 minutes (using a machine capable of giving a force of 1000 G). It will be found that a minimum sludge deposit will result in portions which have been heated to a certain temperature or temperatures and further portions of the sample may then be heated to temperatures at narrower intervals around this temperature or these temperatures in order to find more accurately the elevated temperature range corresponding to low sludging tendency.

The following table gives the results obtained in such determination on a Middle East crude oil.

The tests in the upper half of the table at 15-20" intervals indicate that minimum sludge deposition occurs after heating to a temperature in the region of 145 F.

3,027,319 Patented Mar. 27, 1962 The further tests at 5 intervals around this figure show that the optimum elevated temperature is about F. and that considerable reduction in the sludge forming tendency of the oil occurs if it is heated to a temperature in the range 140-150 F. In this case the upper temperature limit appears to be critical since if the heat treatment temperature is increased to F. practically no reduction in the sludge forming tendency occurs at all.

The carrying out of the process economically on a large scale presents considerable difiiculties. The most obvious source of power for the process is the natural gas separated from the crude oil at the well-head and the process should in any case be carried out in the oil field areas if the formation of sludge in ships tanks and land-based storage vessels is to be avoided. However, in many oil-producing countries, for example the Middle Eastern countries, the separated crude oil is, during all or most of the year, at a higher temperature than 70 P. which is the preferred upper limit of the low temperature. The oil has therefore, in most cases, to be initially cooled before it is heated to the elevated temperature. It is also essential that the oil be cooled, after heating to the elevated temperature, to a temperature at which it is suitable for transportation or atmospheric storage. Furthermore, the provision of furnaces for heating large quantities of oil represents a considerable capital expenditure.

Thus, although the process can be carried out easily enough on a small scale, it has not hitherto been possible to operate it on an economic basis on a large scale.

We have now developed a method of carrying out the process, which can be operated economically on a large scale and in which the whole of the power requirements may, if desired, be derived from the natural gases separated from the crude oil in the oil fields.

According to the invention, a process for treating crude oil so as to reduce its sludge-forming tendency comprises heating the oil from the low temperature to the elevated temperature by dissolving in the oil compressed vapors which have been produced by auto-refrigeration of the oil.

Where the oil from oil wells or gas separators is originally at the low temperature, it may be heated to the elevated temperature by dissolving in it compressed vapors obtained by subsequently auto-refrigerating the heated oil so as to cool it to its original temperature. As has been previously stated, it is essential to cool the oil finally to the temperature at which it is in a suitable condition for subsequent storage and transportation.

Where the oil is initially at a higher temperature than the low temperature, a condition which frequently occurs in Middle Eastern oil fields where the ambient temperature is at most times above 70 F. and frequently above 100 F., it may first of all be cooled to the low temperature by auto-refrigeration, the vapors produced in this step being compressed and re-dissolved in the oil so as to provide some or all of the heat necessary for heating it to the elevated temperature.

The auto-refrigeration of the oil may be carried out in flash vessels operating under vacuum. The vacuum in the auto-refrigerators may advantageously be maintainedby means of axial-flow compressors with interstage cooling. Some or all of the compressed Vapors may be passed through indirect heat exchangers where some of the heat of the vapors is given up to water which circulates through a cooling tower.

The compressors and the pumps required for circulating the oil through the system may be conveniently operated by gas turbines. Natural gas separated from the crude oil at the well-head may be used to supply the power for operating the compressors and pumps.

The invention will now be described by way of example with reference to the accompanying drawing.

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It will besassumed that the plant is being used fortreating a separated Middle Eastern crude oil under summer conditions with an ambient temperature in the region of 100 F., and that, in order to achieve good results, it is desired to cool the oil to 70 F. before heating it to the elevated temperature which, it will be assumed, is 140- 150' F.

Crude oil from a well-head gas separator enters by line and is pumped by pump 11 into the primary cooler 12 which is equivalent to a single equilibrium flash stage. Cooler 12 is a vertically mounted cylindrical vessel containing a packed section 13 which, in order to achieve a low pressure drop, consists of a six-foot depth of serrated three-inch slats or other suitable packing. Cooling of the oil in cooler 12 is achieved by auto-refrigeration, a partial vacuum being maintained by compressor 14. The pressure in cooler 12 will have to be adjusted to somewhere below 760 mm. Hg abs., according to the temperature of the incoming crude oil, so as to cool the oil to 70 F. The cooled oil is pumped from cooler 12, along line 15, by means of pump 16. Vapors produced during the auto-refrigeration of the oil in cooler 12 are drawn ofl? along line 36 and are compressed by the axial-flow compressor 14 which consists of a number of stages with inter-stage cooling. The compressor is capable of passing varying quantities of vapor at varying suction pressures. The compressed vapors leaving compressor 14 pass along valved lines 17 and 13 and are then redissolved in the oil passing along line 15. The vapors passing along line 18 pass through heat exchanger 19 where they are cooled and partly condensed by indirect heat exchange with cooling water. Valves 20 and 21 enable the quantities of vapor passing through lines 17 and 18 to be ad justed, whereby the amount of heat in the vapors which are passed into solution in the oil in line 15 may be adjusted so as to raise the temperature of the oil by the desired amount. It is convenient at this stage to raise the temperature of the oil by 10 F, i.e. from 70 F. to

The oil in line 15 is further heated to 140150 F. by dissolving in it vapors from line 22 which are produced by auto-refrigeration of the oil in secondary coolers 23 and 24 which are equivalent to a single equilibrium flash stage. In coolers 23 and 24 the oil is cooled from 140- 150 F. to the temperature at which it entered the plant,

i.e. the temperature at which it left the separators. The

coolers 23 and 24 operate in parallel and are similar in construction to cooler 12. They are maintained under vacuum by means of compressors 25; the actual operating pressure in coolers 23 and 24 is adjusted in accordance with the degree of cooling required. The vapors produced during the auto-refrigeration of the oil in coolers 23 and 24 are compressed by the axial-flow compressor 25 which consists of a number of stages with interstage cooling andwhich is capable of passing varying quantities of vapor at varying suction pressures. The compressed vapors leaving compressor 25 pass along valved lines 26 and 27 and are then passed into solution in the oil passing along line 15. The vapors passing along line 26 are cooled and partly condensed in heat exchanger 28 and the valves 25! and 30 enable the quantities of vapor passing through lines 26 and 27 to be adjusted, whereby the amount of heat in the vapors which are passed into solution in the oil in line 15 may be adjusted so as to raise the temperature of the oil to 140150 F.

Cooling water for the heat exchangers 19 and 28 is circulated by pump 31 through a cooling tower 32.

The treated oil is pumped from coolers 23 and 24 by means of pumps 33 and 34 and leaves the plant by line 35 at or below the temperature at which it entered by line 10.

The plant described is suitable for treating almost any crude oil. 'It will be evident that where the oil entering the plant is already below 70 F., the primary cooling stage may be by-passed.

We claim:

1. A continuous process for treating crude oil to reduce its tendency to deposit sludge on storage comprising heating the oil from a temperature not higher than about 70 F. to an elevated temperature above F. by dissolving compressed vapors therein which have been obtained from said oil, said maximum elevated temperature being that temperature at which minimum sludge deposition, by test, is deposited, subjecting the heated oil to auto-refrigeration to produce oil vapors therefrom, compressing said produced vapors, and passing the compressed vapors to the oil undergoing treatment for dissolution therein to heat same.

2. A process according to claim 1, in which the autorefrigeration is carried out in flash vessels maintained under vacuum by means of axial-flow compressors with inter-stage cooling.

3. A process according to claim 1, in which natural gas separated from the crude oil at the well-head is used to supply the power requirements of the process.

4. A continuous process for treating crude oil so as to reduce its tendency to deposit sludge on storage comprising heating the oil from a temperature not higher than about 70 F. to an elevated temperature above 100 F. and, not in excess of about F. by dissolving compressed vapors therein which have been obtained from said oil, subjecting the heated oil to auto-refrigeration to produce oil vapors therefrom, compressing said produced vapors, said passing the compressed Vapors to the oil undergoing treatment for dissolution therein to heat same.

5. A continuous process for treating crude oil so as to reduce its tendency to deposit sludge on storage comprising cooling the oil to a temperature of about 70 F. by auto-refrigeration, heating the cooled oil to an elevated temperature by dissolving compressed vapors therein which have been obtained from said oil, said maximum elevated temperature being that temperature at which minimum sludge deposition, by test, is deposited, subjecting the heated oil to auto-refrigeration to produce oil vapors therefrom, compressing said produced vapors, and passing the compressed vapors to the oil undergoing treatment for dissolution therein to heat same.

6. A continuous process for treating crude oil so as to reduce its tendency to deposit sludge on storage comprising cooling the oil to a temperature of about 70 F. by auto-refrigeration, heating the cooled oil to an elevated temperature not in excess of about 150 F. by dissolving compressed vapors therein which have been obtained from said oil, subjecting the heated oil to auto-refrigeration to produce oil vapors therefrom, compressing said produced vapors, passing the compressed vapors to the oil undergoing treatment for dissolution therein to heat same.

7. A continuous process for treating crude oil having a temperature higher than the minimum temperature value desired so as to reduce its tendency to deposit sludge on storage, comprising cooling the oil to a temperature below about 70 F- by auto-refrigeration, heating the cooled oil to an elevated temperature above 100 F. and not in excess of about 150 F. by dissolving compressed vapors therein which have been obtained from said oil, cooling the heated oil to its original temperature by autorefrigeration to produce oil vapors therefrom, compressing the vapors produced by the auto-refrigeration steps, and passing same for dissolution in the oil after the first cooling step to provide at least some of theheat required for heating the oil from the low temperature to the elevated temperature.

References Cited in the file ofthis patent UNITED STATES PATENTS 1,953,353 Lazar et al. Apr. 3, 1934 2,048,371 Calderwood July 21, 1936 2,249,884 Carney July 22, 1941 2,315,935 Child Apr. 6, 1943 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION P jfiirt N0. 3.027.319 March 27, 1962 Percy Meyer et a1.

numbered pat- It is hereby certified that error appears in the above ant requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 29, for "said" read and Signed and sealed this 14th day of August 1962..

DAVID L. LADD E ,tfi w. SWIDER Commissioner of Patents Atteet i r xg Officer 

1. A CONTINUOUS PROCESS FOR TREATING CRUDE OIL TO REDUCE ITS TENDENCY TO DEPOSIT SLUDGE ON STORAGE COMPRISING HEATING THE OIL FROM A TEMPERATURE NOT HIGHER THAN ABOUT 70* F. TO AN ELEVATED TEMPERATURE ABOVE 100*F. BY DISSOLVING COMPRESSED VAPORS THEREIN WHICH HAVE BEEN OBTAINED FROM SAID OIL, SAID MAXIMUM ELEVATED TEMPERATURE BEING THAT TEMPERATURE AT WHICH MINIMUM SLUDGE DEPOSITION, BY TEST, IS DEPOSITED, SUBJECTING THE HEATED OIL TO AUTO-REFRIGERATION TO PRODUCE OIL VAPORS, THEREFROM, COMPRESSING SAID PRODUCED VAPORS, AND PASSING THE COMPRESSED VAPORS TO THE OIL UNDERGOING TREATMENT FOR DISSOLUTION THEREIN TO HEAT SAME. 